1. Field of the Invention
The present invention relates to a digital television relay apparatus, and more particularly, to a linearization compensation system of a digital TV relay apparatus for compensating a non-linearity of a high power amplifier, and methods thereof.
2. Background of the Related Art
In general, a digital TV relay apparatus services a radio shadow area that a digital broadcast transmitter fails to cover. Thus, a digital TV relay improves the viewing range of a digital TV broadcast service.
A receiving terminal of the digital TV relay apparatus down-converts a broadcast signal transmitted from a digital TV transmitter into an intermediate frequency and transmits it to a transmitting terminal of the digital TV relay. Then, the transmitting terminal modulates the intermediate frequency to a channel frequency, amplifies it to a predetermined level and transmits it to a general subscriber located in the radio shadow area.
The broadcast signal outputted from the transmitting terminal of the digital TV relay is a signal that has been amplified by a high power amplifier. Typically, the output of a high power amplifier is non-linear over a range of input voltages at a given frequency.
FIG. 1 is a schematic block diagram of a linearization compensation system of a digital TV relay apparatus in accordance with the related art. As shown therein, the linearization compensation system of the digital TV relay apparatus includes a transmitting unit 10 to modulate an image data into an intermediate frequency (IF) signal, amplify the intermediate frequency signal by a high power amplifier (HPA) and transmit it. The linearizer further includes a vector signal analyzer 20 to optimize the output signal of the HPA and generate an equalizer coefficient. The linearizer also includes a computer 30 to receive the equalizer coefficient from the vector signal analyzer 20 and generate a linear compensation coefficient using internal linear compensation software and output the linear compensation coefficient to the modulator of the transmitting unit 10.
The transmitting unit 10 includes a modulator 1 to modulate an image data to an IF signal according to the linear compensation coefficient outputted from the computer 30. The transmitting unit 10 also includes an up-converter 2 to convert the IF signal outputted from the modulator 1 into a radio frequency (RF) signal, and an HPA 3 to amplify the RF signal outputted from the up-converter 2 and output it through an antenna 4.
The modulator 1 includes a digital filter 1-1 to remove an interference through a channel coding, insert a pilot signal and generate a waveform.
The vector signal analyzer 20 includes a down-converter 6 to convert the output signal of the HPA 3 into an IF signal, and a demodulator 7 to demodulate the IF signal outputted from the down-converter 6 into a digital I/Q baseband signal. The demodulator 7 includes an adaptive equalizer 7-1 to generate an equalizer coefficient of the digital I/Q signal by using an adaptive equalization algorithm.
The operation of the related art linear compensation system of a digital TV relay apparatus constructed as described above will now be described with reference to FIG. 1. When broadcast data is inputted, the modulator 1 modulates a baseband broadcast signal of an MPEG stream into an IF signal. The digital filter 1-1 of the modulator 1 distorts the broadcast signal by using an initial linear compensation coefficient so that the broadcast signal may have an inverse characteristic to the non-linear characteristic of the HPA 3.
The up converter 2 converts the IF signal outputted from the modulator 1 into a broadcast frequency, that is, an RF signal, which is determined by broadcasting stations. Accordingly, the RF signal is amplified to a prescribed level by the HPA 3 and transmitted as a digital TV signal through the antenna 4 to a general subscriber.
If the modulator 1 is ideally operated, the characteristic of the digital filter 1-1 and the non-linear characteristic of the HPA 3 would be offset, and an output signal of the HPA 3 would not have a distortion component. However, it is not possible for the digital filter 1-1 to have the exact inverse characteristic to the non-linear characteristic of the HPA 3. Thus, a distortion component is included in the output signal of the HPA 3. Accordingly, an operator who operates the transmitter and the relay apparatus must compensate for the non-linearity of the digital TV signal outputted through the HPA 3 by using the vector signal analyzer 20 and the computer 30.
When the operator operates the linear compensation units (the vector signal analyzer 20 and the computer 30), the RF signal outputted from the HPA 3 is inputted through a coupler 5 to the vector signal analyzer 20. The down-converter 6 of the vector signal analyzer 20 converts the input RF signal into an IF signal and outputs the IF signal the demodulator 7. Then, the demodulator 7 converts the inputted IF signal into a digital I/Q signal of a baseband and inputs it to the adaptive equalizer 7-1. The adaptive equalizer 7-1 generates an equalizer coefficient to optimize the digital I/Q signal by using the adaptive equalization algorithm.
The adaptive equalization algorithm performs optimizing by using a fixed convergence constant (a step size) set by the operator. Accordingly, as the digital I/Q signal is processed by the adaptive equalizer 7-1, a signal-to-noise ratio (SNR) of the digital I/Q signal is improved. An operator may visually confirm the optimization of the digital I/Q signal using the vector signal analyzer 20.
After the optimization of the digital signal I/Q is confirmed, the operator clicks a prescribed button to transmit the 31 coefficients of the adaptive equalizer through a general purpose interface bus (GPIB) to the computer 30. The computer 30 converts the received 31 equalizer coefficients into 288 linear compensation coefficients by using the linear compensation software 8 and outputs them through the RS-232C cable to the digital filter 1-1 of the modulator 1.
The digital filter 1-1 distorts the broadcast signal by using the changed linear compensation coefficients outputted from the computer 30, so that a clear digital TV signal without distortion is transmitted through the antenna 4.
The apparatus and method of the related art have various problems. For example, in the related art linear compensation system, the adaptive equalizer 7-1 performs the adaptive equalization algorithm by using the fixed convergence constant (the step size) selected by the operator. However, because the operator can not accurately determine the convergence speed of the adaptive equalizer according to the value of the step size, he or she repeatedly changes the step size value. As a result, the convergence (or optimization) time is very long in the related art linear compensation system of a digital TV relay apparatus. In addition, in the related art linear compensation system of a digital TV relay apparatus, the operator must change the step size value by increments of one. Each time, the equalizer coefficient must be transmitted to the computer 30. Moreover, in the related art linear compensation system of a digital TV relay apparatus, an expensive vector signal analyzer 20 is required.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.